Method and means for stabilizing amplifiers



- 1,646,027 o 1927' E. H. LANGE METHOD AND MEANS FOR smarmzme AMPLIFIERS Filed March 27. 1926 6 L F Outpuw Impedan e IN VEN TOR Input Impedance Patented Oct. 18, 1927.

EDWARD H- LANGE, OF BALTIMORE, MARYLAND.

METHOD AND MEANS FOR STABILIZINGv AMPLIFIERS.

Application filed March 27, 1926. Serial No. 97,916.

This invention relates to thermionic amplifiers, and the object of the-invention is to provide simple method and means whereby the introduction of energy into the input circuit of a thermionic amplifier from the output circuit of the amplifier, by way of the inter-electrode capacity of the thermionic tube or tubes, may be prevented. When an' amplifier which is intended to amplify over a band of frequencies is operated at the higher frequencies of its range, the energy transferred from the output circuit to the input circuit is usually greatest, and decreases toward the low frequency end of the range. Due to the mutual capacity of the output and input circuits, even though the magnetic mutual reactance is separately balanced and thereby eliminated, the energy introduced into the input circuit from the output circuit may be sufficient to cause selfsustained oscillations of decreasing intensity toward the low frequency end of the range and relatively great intensity at the highcircuit which suffices to establish a balance over a range of frequencies. The bridge circuit consists of three impedance arms con nected to form a Y,- two of the arms containing capacitive reactance and one containing inductive reactance, the three terminals of the Y being conmctectoneto the grid, one to the plate and one to the filament. By means of this arrangement, which is hereinafter fully described, the two arms of the circuit which are connected between the grid and filament have opposing electromotive forces established across them which give a resultant potential diiference of substantially zero between the grid and the filament due to the output circuit currents.

Figure 1 shows in diagram form an amplifier having means for preventing transfer of energy from the output circuit to the input circuit, and shows the generalized form of the impedance bridge.

Figure 2 illustrates diagrammatically the electric circuit equivalent of one of the stages of Figure 1.

Figure 3 shows the bridge circuit of Figure 2 with the inductive branch connected to the filament.

Figure 4 shows the bridge circuit of Figure 2 with the inductive branch connected to the grid.

Figure 5 shows the bridge circuit of F igure 2 with the inductive branch connected to the plate.

Figure 6 shows a-vector diagram of the balanced circuit connected as'in Figure 3, and the phase relations of the voltages across the bridge arms.

Figure 7 shows a vector diagram of the balanced circuit connected as in Figure 4, and the phase relations of the voltages across the bridge arms.

Figure 8 shows a vector diagram of the balanced circuit connected as in Figure 5, and the phase relations of the voltages across the bridge arms.

Referring to Figure 1, a radio frequency electromotive force to be amplified is impressed upon the input circuit 2, 3, either directly or by means of the antenna 1. The inherent capacitive coupling between the grid and the plate is shown at 4 for the first stage, and at 14 for the second stage. The

primary of the interstage coupling transformer 11 is connected as shown to the source of plate supply +B, likewise the primary 16 of the second stage is connected to +B, and the negative terminal B of the plate supply connected to the negative terminal A of the filament or cathode current supply. The bridge impedances are shown at 5, 1.0, and 8, 5 being the grid branch, 10 the plate branch, and 8 the filament branch. A similar bridge circuit is shown connected to the grid. plate and filament of the second stage. The output of the first stage is coupled to the input circuit 12, 13, of second stage as shown. Although the coupling shown between the first and second stages is inductive. the balancing principle of circuits 5, 8, and 10 is independent of the nature of the input or the output circuits, and Figure 1 illustrates only one of numerous circuits to which the bridge may be applied.

Referring to Figure 2, the grid, plate and filament of a stage of a thermionic amplifier are indicated by G, P and F respectively. The internal impedance of the tube between the plate and the filament is shown by 13,, and the inherent capacity shown by the condenser between G and P. An alternating potential difference of magnitude E impressed upon the grid, between the filament and grid, establishes in the output circuit an electro-motive force of magnitude ,aE the output circuit consisting of the tube impedance n and the output impedance in series. This invention deals with a method of preventing the establishment of a potential difference across the points GF due to the reaction of the potential difference which ex ists across the points PF of the output circuit through the inherent capacity between GP. By means of two capacitive reactances and an inductive reactance arranged as shown at Z Z and Z and connected to a common point 0, a potential difference E is established in opposition to the potential difference E and this condition can be established by any one of three arrangements of the inductive reactance in any of the three arms, and for any one arrangement by a suitable proportioning of the values of capacity and inductance.

Figure 3 shows a specific arrangement of the above generalized form of bridge, in which the inductive reactance is connected in the filament branch. The resultant reactance of the parallel circuit consisting of condensers across the points PO is capacitive, the potential differences across the elements PO, GO, and GP therefore lag the currents through the branches by substantially 90 degrees. WVhen the circuit is balanced, that is, when the resultant potential difference across GF is zero, so far as an potential difference established across G by the output circuit is concerned, the total current of the parallel circuit consisting of the three condensers GP, GO,PO flows from the junction point 0 through the inductive reactance OF, and the potential difference across OF leads this current by substantially 90 degrees. The potential difference across G0 is therefore substantially 180 degrees out of phase with the potential difference across FO. By adjusting the values of capacity and inductance so that the magnitude of the voltage drops across the branches conforms to the relation the potential difference across PF, that is the output potential difference, has no effect in producing a potential difference across the input GF, and the alternating current in the output circuit is due solely to the impressed signal E which sets up an electromotive force of value ,aE in the output circuit, a being the amplification factor of the thermionic tube. The current and voltage relations existing in the balanced bridge circuit can be still better understood by reference to Figure 6 which shows in vector form the balance conditions. E is the voltage across the parallel circuit consisting of the three condensers, that is, the two bridge condensers and the inter-electrode capacity, and the portion E of this voltage shown, is in phase with the voltage E I is the current through the branch PO, which leads the voltage E by substantially 90 degrees, also 1 is the current through the branch PGO, which leads the voltage E, by substantially 90 degrees. The total current I plus I which equals L flows through the branch F0, and since in this instance the branch FO consists of an inductive reactance, the voltage drop E is 90 degrees ahead of the current L that is the current L lags the voltage E by substantially 90 degrees. The resultant voltage E established by output circuit currents flowing in the bridge arms is therefore zero.

Figure 4:, and the corresponding vector diagram of Figure 7, show the operation of the circuit when the grid branch of the bridge contains the inductive reactance. In Figure '7, E is the voltage across the parallel circuit consisting of the condenser P0 in parallel with the branch PGO. which consists of the inter-electrode capacity and inductive reactance GO in series. The inductive reactance is adjusted at the highest frequencies of the range to be covered to be less than the capacitive reactance of the capacity GP, so that this condition will necessarily prevail at all lower frequencies. The resultant reactance of the branches PG and GO in series is therefore always capacitive, and the current through this part of the parallel circuit leads the voltage E by substantially 90 degrees. This current is shown by I and since this current. passes through the inductive reactance, the voltage across the arm GO leads the current 1, by substantially 90 degrees. The total current L; plus 1 passes through the condenser F0. and the voltage F lags the total current L by substantially 90 degrees, thereby opposing the voltage E and the voltage E O.

Figure 5, and the corresponding vector diagram of Figure 8 illustrate how the circuit functions to preventv the transfer of energy from the output to the input circuit when the plate branch contains the inductive reactance. In Figure 5, the parallel circuit across PO consists of the inductive reactance P0 in parallel with the branch PGO formed b the inherent capacity and the condenser O in series. The reactance of the condensers PGO are made greater than the reactance PO at the higher frequencies to be covered, so that this condition will prevail for all lower frequencies, and the current in the inductive arm PO is always in excess of the current in the capacitive branch PGO.

In Figure 8, E shows the voltage across PO, the current I lagging E by substantially 90 degrees. The current I through the condenser arm PGO leads the voltage E by substantially 90 degrees, and the voltage lfi lags the current l, by substantially 90 degrees. The total current I lags the volt.- age E by substantially 90 degrees and since this current flows through the condenser Ft), it produces a voltage E lagging substantially 90 degrees the current I and therefore in phase opposition to E In each of the vector diagrams shown the sense of rotation of the vectors is counterclockwise, as shown. The relation previously given among the magnitudes of the voltages across the arms when a balance exists, that is the relation between E E E and E,,,,, holds for any and all of the. specific forms of circuit described, the sign of the voltage E being negative when the grid arm contains an inductive reactance.

In the balancing arrangements above described a separate balancing circuit consisting of two arms with capacitive reactancc and one with inductive reactance are shown in addition to the input and output circuits which may consist of a variety of ditl'erent types of circuit and inter-stage couplin This invention contemplates the use either of the input or the output circuit as a substitute for the reactance of one of the arms of the balancing circuit, also the use of mu tual capacity within a thermionic device as a substitute for the two capacity arms of the balancing circuit, as for example the mutual capacity between an additional electrode and the grid and plate of a thern'iionic tube having a cathode, grid, plate and additional electrode.

In similar manner it will be under ;tood that many other modifications and changes may be made in the specific embodiment of my invention and Within the spirit of the appended claims.

What is claimed is:

1. The circuit connections transfer of energy from a first circuit carrying a hi h frequency alternating current to a secon circuit having a conductive connection With the first circuit and inherent electrostatic coupling with the first circuit,

which consist of bridging the ternunals of the inherent electrostatic coupling by means of a capacitive coupling, and connecting an intermediate point of said capacitive con,- pling by means of an inductance to said conductive connect-ion, thereby opposing the portion of the voltage across the first circuit between the conductive connection and said intermediate point to a portion of the voltage of opposite phase across said capacitive coupling, and producing a resultant voltage of zero in the second circuit between the terminal of said inherent electrostatic preventing coupling and conductive connection, due to alternating currents in the first circuit.

2. The method of preventing transfer of energy from a first circuit carrying a high frequency alternating current to a second circuit having a conductive connection with the first circuit and inherent electrostatic coupling with thefirstcircuit, which consists of dividing the voltage across the inherent electrostatic coupling into two parts, and opposing against one (if said parts between the point of division and said conductive connection, an equal voltage established by the series relation of the said divided electrostatic coupling and common conductive connection across the first circuit, thereby annulling the voltage established across the second circuit due to inherent electrottatic coupling with the first high frequency alternating current carrying circuit.

3. In athermionic amplifier having a cathode, plate and grid, and inherent ca pacity between the plate and the grid, an alternating current input circuit and an alternating current output circuit, the circuit connections comprising three impedances connected from the cathode, plate, and grid, to a common junction, establishing between the cathode and the plate a series-parallel circuit whereby, due to the flow of a portion of the output alternating current in said impedances and inherent capacity, two equal voltages in phase opposition are established between the cathode and the grid, and reamplification of a resultant voltage between the grid and cathode due to a voltage between the plate and the cathode prevented.

4. In a thermionic amplifier having an alternating current input circuit connected between the grid and the filament, an alternating current output circuit connected between the plate and the filament, and inherent capacity between the grid and the plate, the method of annulling the voltage set up in the input circuit from the output'circuit, which consists of establishing in series across the alternating current output circuit and independent of said circuit two unequal electromotive forces; in phase opposition and impressing the smaller of the electromotive forces upon the grid in opposition to the electroi'uotive force impressed upon the grid through said inherent capacity from the plate, thereby producing a resultant of zero voltage between the filament and grid.

5. In an amplifier containing a thermionic tube having a cathode, plate and grid, with inherent impedance between the plate and grid, and having an alternating current input circuit, and an alternating current output circuit. a bridge circuit consisting of one impedance arm connected to the grid, one impedance arm connected to the plate and one impedance arm connected to the cathode, said impedance arms having a common junction and consisting of a condenser in each of two of said arms and an inductance in the third arm, whereby a voltage may be established between the cathode and common junction equal in magnitude and opposite in phase to the voltage established between the common junction and the grid, said voltages being established by the alternating voltage across the output circuit impressed upon two of said arms directly and one of said arms through said inherent impedance.

6. In a thermionic amplifier having a filament, plate and grid, and inherent capacity between the grid and the plate,'an alternating current input circuit and an alternating current output circuit, an impedance network formed by two condensers and an inductance connected from the grid, plate and filament to a common junction, whereby the voltage established between the filament and the common junction may be opposed against the voltage established between the common junction and the grid through the inherent capacity, to produce a resultant voltage of zero between the grid and the filament due to the potential difference across the alternating current output circuit.

7. In a system of two electric circuits having a common connection and inherent electrostatic coupling between a terminal of each of the circuits, a network of impedances comprising three arms connected from a neutral point to the said common connec tion and to the terminals of the inherent electrostatic coupling of the first and second circuits. whereby the transfer of energy from the first circuit to the second circuit through the inherent electrostatic coupling may be prevented by opposing the voltage established between the common connection and neutral point to the voltage established between the neutral point and the terminal of the inherent electrostatic coupling of the second circuit, said opposing voltages being established by the flow of alternating current from the first circuit through the said three arms and inherent electrostatic coupling.

8. In a system of two electric circuits having one common point and inherent capacitive coupling with each other between a terminal of the first circuit and a terminal ot the second circuit, a network of three impedance arms connected from the common point and each of the terminals of the inherent capacitive coupling to a neutral point, two of said arms containing capacitive reactance and the third containing inductive reactance, whereby the transfer of energy from the first circuit to the second circuit through the said inherent capacitive coupling may be prevented by establishing across two of said arms between the common point and terminal of the said inherent capacitive coupling of the second circuit, two equal voltage-s in phase opposition derived by the flow of a portion oi the alternating current of the first circuit through said impedance arms and said inheient coupling.

9. In a thermionic amplifier having a cathode, plate and grid, and inherent capacity between the plate and the grid, an alternating current input circuit and an alternating current output circuit. circuit connections comprising three impedances connected from the cathode, plate and grid to a common point, whereby a portion of the output alternating current may be combined at said common point with another portion of the output alternating current carried by said inherent capacity, and two voltages in phase opposition established between the filament and grid to maintain a resultant of zero voltage between the filament and grid due to alternating current in the output circuit.

In witness whereof I have hereunto set 'my hand this 24th day of March, 1926.

EDWARD H. LANGE. 

